Internal combustion engine with spark ignition and direct fuel injection comprising a very high-pressure direct injection system

ABSTRACT

The invention relates to an internal combustion engine with spark ignition and direct fuel injection. The inventive engine comprises: a cylinder; a cylinder head; a piston which is mounted to slide in the cylinder; a combustion chamber which is defined in the cylinder between the cylinder head and the piston; a fuel injector which opens into the combustion chamber; a spark plug; inlet and exhaust valves; and a fuel injection pump which supplies the injector with a flow of pressurized fuel. According to the invention, the pressure of the fuel flow supplied to the injector exceeds 300 bar and, preferably, reaches or exceeds 500 bar.

The present invention relates generally to injection techniques forgasoline engines with direct injection.

More precisely, the invention relates to an internal combustion enginewith spark ignition and with direct fuel injection, this enginecomprising at least a cylinder elongated along an axis, a head whichcloses off the cylinder in a fixed manner, a piston mounted to slide ina sealed manner in the cylinder along the axis of the cylinder, acombustion chamber defined in the cylinder between a lower surface ofthe head and an upper surface of the piston, a fuel injector connectedto the head and opening into the combustion chamber, a spark plugconnected to the head and having electrodes which selectively produce aspark in the combustion chamber, intake and exhaust valves, movablymounted in the head, which are arranged on both sides of an axialmid-plane of the cylinder and which selectively close off the combustionchamber, and an injection pump which selectively supplies the injectorwith a flow of pressurized gasoline.

Although this definition uses very concrete and precise terms, such asinjector, spark plug, or spark, for the sake of clarity, it should beunderstood that these terms cover their technical equivalents, the wordsmentioned by way of example thus being respectively synonymous withinjection means, ignition means, and ignition point.

The engines thus defined are well known to the expert in the field.

The engines with spark ignition and direct fuel injection offer thepossibility of optimized functioning with so-called “weak” mixtures,that is to say carbureted mixtures containing a very great excess ofair, relative to stoichiometric conditions, with respect to the quantityof fuel introduced into the cylinder.

These engines not only offer great potential in terms of reduction offuel consumption, but also they participate in environmental protectioninasmuch as the excess of air makes possible complete combustion of thefuel, and thus prevents release of unburned fractions in the exhaustgases.

The carbureted mixture which results from injection of fuel into thecylinder can be a homogeneous mixture or a stratified mixture in whichthe air/fuel ratio is not uniform in the whole cylinder, this lattercase leading to localization near the ignition point of the carburetedmixture in a flammable air/fuel ratio.

Different solutions are known for producing a stratified mixture usingdirect fuel injection.

A first solution consists of producing wall effect stratification bydirecting the injection jet towards a wall, for example, in thedirection of the upper surface of the piston, and then deflecting ittowards the spark plug. This solution can have the disadvantage ofleading to the formation on the walls of a film of liquid fuel whichburns and produces soot. Furthermore, this solution does not makeoptimal stratification possible because the impact of the jet on thewall leads to excessive scattering.

A second solution consists of producing aerodynamic effectstratification by giving the air a well-defined movement so as to directthe fuel projected by the injector towards the spark plug. However, thelocalization of the fuel around the spark plug varies greatly from onecycle to another because of the random and turbulent nature of the airflow, which is detrimental to the stability of the engine and makes itdifficult to regulate.

The third solution consists of positioning the injector and the sparkplug in such a way that the fuel jet is projected by the injectordirectly towards the spark plug. This process is a priori veryeffective, but can be sensitive to the uncertainties affecting thepositioning and to the characteristics of the fuel jet. Production ofthe stratified mixture results from injection of the fuel a short timebefore triggering of the combustion. Unlike in the preceding twosolutions, the time available under these conditions for injection,atomization, vaporization of the gasoline, and preparation of themixture before triggering combustion is very often insufficient. Theliquid fuel, present during combustion and difficult to ignite, thencauses fouling of the spark plug.

Another technique consists of assisting the fuel injection by means ofan auxiliary injection of air. Although this process leads to relativelyfine atomization of the fuel, it on the other hand requires the use ofan additional system for the air compression.

Another known injection technique, described in the patent U.S. Pat. No.5,992,353, consists of proceeding with injection of the fuel, in thevapor state mixed with superheated steam, at a pressure at least equalto 350 bar. This solution is in concrete terms inapplicable toautomobiles, inasmuch as it requires on board an auxiliary water tankand a heat exchanger using the exhaust gases to vaporize the fuel andobtain the superheated vapor.

In this context, the present invention aims to offer an engine which, inspite of a relatively simple structure, operates with a very lowproduction of soot, with a more rapid and reproducible combustion offuel, and without fouling the injector holes.

For this purpose, the engine of the invention, which in other respectsis in accordance with the generic definition given for it in thepreamble above, is essentially characterized by the fact that thepressure of the flow of gasoline provided to the injector exceeds 300bar, and is preferably at least equal to approximately 500 bar.

Thanks to this injection pressure, the duration of the injection of fuelinto the combustion chamber can be reduced because of the increase ofthe static delivery. Moreover, the time thus gained can be used toadvantage for the physical realization of atomization, evaporation andmixing of the fuel with the air.

The liquid fuel injected at this high pressure undergoes a primaryatomization which breaks the jet into large drops, and then a secondaryatomization which breaks these large drops into very fine drops. What iscalled the “break up” (that is to say fractionation) length, whichcharacterizes the length of the jet necessary for realization of theprimary atomization, is reduced by the increase of the speed ofdischarge of the fuel from the injector, which increases with theinjection pressure.

It has appeared that for pressures greater than 300 bar, thefractionation length becomes negligible. The fuel then comes directlyout of the injector in the state of a mist of droplets whose secondaryatomization is finer. The jet is wider, which promotes mixing with theair contained in the combustion chamber and vaporization of the fuel.The finer atomization of the fuel makes faster vaporization possible andenables one to obtain an entirely evaporated and mixed air/fuel mixtureat the time of combustion triggering. The mixture obtained is morehomogeneous, burns faster, and produces less soot. The quantity ofliquid fuel which reaches the spark plug is reduced, which reducesfouling of the spark plug. The increase of the speed of combustionallows triggering combustion closer to top dead center of the piston inthe cylinder, and therefore an increase of efficiency of the cycle. Thereduction of the pollutants and soot makes possible a considerableincrease of the combustion efficiency.

Furthermore, the time gained in execution of each of the phases ofinjection, atomization, and vaporization makes it possible to injectlater during the compression phase, and therefore under higher pressureconditions, which promotes reduction of penetration of the jet, and at ahigher temperature, which promotes vaporization.

In an advantageous embodiment of the invention, a recess can be formedin the upper surface of the piston in order to ensure containment of thegasoline injected into the combustion chamber.

As for the lower surface of the head, it can be shaped like a roof andhave a crest in the axial mid-plane of the cylinder.

Preferably, the injector is closer to the axis of the cylinder than isthe spark plug, and for example, is arranged on the axis of the cylinderitself.

The injector and the spark plug are advantageously separated by adistance at least equal to 5 millimeters and at most equal to 30millimeters.

The injector can inject the gasoline into the combustion chamber roughlyin the shape of an injection cone with a cone angle at least equal to 40degrees and at most equal to 100 degrees, with it being possible forthis cone to be formed by a single jet, such as a hollow and continuousjet, or by several distinct jets, and for example, two to twelve jets.

The injector and the spark produced by the spark plug can be separatedby a distance at least equal to 10 millimeters and at most equal to 30millimeters, while the injection cone and the sparks of the spark plugare, for example, separated by a distance at least equal to 1 millimeterand at most equal to 10 millimeters.

The engine of the invention preferably has two intake valves on a firstside of the axial mid-plane of the cylinder and two exhaust valves on asecond side of the axial mid-plane of the cylinder, with it beingpossible for the spark plug to be arranged between the two intakevalves, or between an intake valve and an exhaust valve.

Other characteristics and advantages of the invention will emergeclearly from the description given for it in the following, in anindicative and in no way limiting basis, with reference to the appendeddrawings, in which:

FIG. 1 is a partial schematic section of an engine according to theinvention;

FIG. 2 is a view from below, on a reduced scale, of the lower surface ofthe head of the engine illustrated in FIG. 1; and

FIG. 3 is a view similar to FIG. 2, illustrating an embodiment variant.

As stated in the preceding, the invention relates to an internalcombustion engine with spark ignition and with direct fuel injection.

This engine, in a non-limiting manner and in a known manner (FIG. 1),has cylinder 1, head 2, piston 3, combustion chamber 4, fuel injector 5,spark plug 6, one or preferably two intake valves 71, one or preferablytwo exhaust valves 72, and injection pump 8.

Cylinder 1 is elongated along its longitudinal axis Z and is closed offat one of its ends by head 2 which is attached to cylinder 1.

Piston 3 is mounted so as to slide in a sealed manner in cylinder 1along axis Z and closes off the other end of this cylinder.

Combustion chamber 4 is thus defined in cylinder 1 between lower surface20 of head 2 and upper surface 30 of piston 3.

Fuel injector 5 is connected to head 2 and opens into combustion chamber4.

Spark plug 6 is connected to head 2 and has electrodes 60 which producea spark in combustion chamber 4 when piston 3 is in the vicinity of itstop dead center.

Intake valves 71 and exhaust valves 72 are movably mounted in head 2 andare arranged on both sides of axial mid-plane P of cylinder 1 in such away as to define an intake side and an exhaust side.

The two intake valves 71 are moved by a cam shaft, or controlleddirectly, in such a way as to place combustion chamber 4 incommunication with intake manifold 710 at a chosen instant precedingcompression.

In a similar manner, the two exhaust valves 72 are moved by a cam shaft,or controlled directly, in such a way as to place combustion chamber 4in communication with exhaust manifold 720 at a chosen instant aftercombustion.

Finally, injection pump 8 provides injector 5 with a flow of pressurizedgasoline at a chosen instant during compression.

According to an essential aspect of the invention, the pressure of thegasoline flow provided to injector 5 by pump 8 exceeds 300 bar andpreferably reaches or exceeds 500 bar.

Other characteristics, although less important, can be provided in orderto optimize the effects obtained by the high injection pressureprescribed by the invention, and are listed in the following.

First of all, concave recess 300 can be formed in upper surface 30 ofpiston 3 in order to ensure containment of the gasoline injected intocombustion chamber 4.

Furthermore, lower surface 20 of head 2 can be shaped like a roof andhave crest 21 in axial mid-plane P of cylinder 1.

As shown in FIGS. 2 and 3, crest 21 thus separates head 2 into a partdedicated to intake (on the right in the figures) and in which the twointake valves 71 are provided, and a part dedicated to exhaust (on theleft in the figures) and in which the two exhaust valves 72 areprovided.

Spark plug 6 can then be arranged between the two intake valves 71(FIGS. 1 and 2) or between an intake valve 71 and an exhaust valve 72(FIG. 3).

Injector 5 is arranged closer to axis Z of the cylinder than is sparkplug 6, and preferably on axis Z itself.

Injector 5 and spark plug 6 are separated by a distance advantageouslybetween 5 millimeters and 30 millimeters, the distance between injector5 and the spark produced by spark plug 6 then typically being between 10millimeters and 30 millimeters.

Injector 5 preferably injects gasoline into combustion chamber 4 in theform of a number of distinct jets, between two and twelve jets.

These jets form injection cone 9 (FIG. 1), which has a cone angle A, forexample, between 40 degrees and 100 degrees.

Finally, injection cone 9 and the sparks formed by electrodes 60 of thespark plug are separated by a distance preferably between 1 millimeterand 10 millimeters.

1. An internal combustion engine, with spark ignition and with directfuel injection, comprising at least one cylinder elongated along an axis(Z), a head which closes off the cylinder in a fixed manner, a pistonmounted so as to slide in a sealed manner in the cylinder along the axis(Z) of the cylinder, a combustion chamber defined in the cylinderbetween a lower surface of the head and an upper surface of the piston,a fuel injector connected to the head and opening into the combustionchamber, a spark plug connected to the head and having electrodes whichselectively produce a spark in the combustion chamber, intake andexhaust valves, movably mounted in the head, the valves being arrangedon both sides of an axial mid-plane (P) of the cylinder, and whichselectively close off the combustion chamber, and an injection pumpwhich selectively supplies the injector with a flow of gasoline at apressure exceeding 300 bar, wherein the injector injects the gasolineinto the combustion chamber roughly in the shape of an injection conewith a cone angle (A) at least equal to 40 degrees and at most equal to100 degrees.
 2. An internal combustion engine according to claim 1,wherein the pressure of the flow of gasoline provided to the injector isat least equal to 500 bar.
 3. An internal combustion engine according toclaim 1, wherein a recess is formed in the upper surface of the pistonin order to ensure containment of the gasoline injected into thecombustion chamber.
 4. An internal combustion engine according to claim1, wherein the lower surface of the head is roof-shaped and has a crestin the axial mid-plane (P) of the cylinder.
 5. An internal combustionengine according to claim 1, wherein the injector is closer to the axis(Z) of the cylinder than is the spark plug.
 6. An internal combustionengine according to claim 1, wherein the injector is arranged on theaxis (Z) of the cylinder.
 7. An internal combustion engine according toclaim 1, wherein the injector and the spark plug are separated by adistance at least equal to 5 millimeters and at most equal to 30millimeters.
 8. An internal combustion engine according to claim 1,wherein the injector injects the gasoline into the combustion chamber inthe form of a plurality of distinct jets comprising between two andtwelve jets.
 9. An internal combustion engine according to claim 1,wherein the injector and the spark produced by the spark plug areseparated by a distance at least equal to 10 millimeters and at mostequal to 30 millimeters.
 10. An internal combustion engine according toclaim 1, wherein the injection cone and the spark produced by the sparkplug are separated by a distance at least equal to 1 millimeter and atmost equal to 10 millimeters.
 11. An internal combustion engineaccording to claim 1, comprising two intake valves on a first side ofthe axial mid-plane (P) of the cylinder and two exhaust valves on asecond side of the axial mid-plane (P) of the cylinder, wherein thespark plug is arranged between the two intake valves.
 12. An internalcombustion engine according to claim 1, comprising two intake valves ona first side of the axial mid-plane (P) of the cylinder and two exhaustvalves on a second side of the axial mid-plane (P) of the cylinder,wherein the spark plug is arranged between an intake valve and anexhaust valve.
 13. An internal combustion engine according to claim 3,wherein the recess is concave and has a substantially uniform depth.